Supramolecular networks with electron transfer in two dimensions

ABSTRACT

Organic charge-transfer (CT) co-crystals in a crossed stack system are disclosed. The co-crystals exhibit bidirectional charge transfer interactions where one donor molecule shares electrons with two different acceptors, one acceptor face-to-face and the other edge-to-face. The assembly and charge transfer interaction results in a pleochroic material whereby the optical absorption continuously changes depending on the polarization angle of incident light.

This application is a continuation of and claims priority to and thebenefit of application Ser. No. 13/526,215 filed Jun. 18, 2012 andissued as U.S. Pat. No. 9,443,636 on Sep. 13, 2016, which claimedpriority to and the benefit of application Ser. No. 61/498,262 filedJun. 17, 2011—each of which is incorporated herein by reference in itsentirety.

This invention was made with government support under DE-SC0000989awarded by the Department of Energy. The government has certain rightsin the invention.

FIELD OF THE INVENTION

The present invention relates generally to solution-phase assembly of a2D supramolecular network of charge transfer complexes that form anarchitecture called a “crossed stack” lattice. These crossed stackstructures exhibit bidirectional charge transfer interactions where onedonor molecule shares electrons with two different acceptors: oneacceptor face-to-face and the other edge-to-face. The assembly andcharge transfer interaction results in a pleochroic material whereby theoptical absorption continuously changes depending on the polarizationangle of incident light. Thus, these crossed stack materials can be usedin various optical applications.

BACKGROUND OF THE INVENTION

The field of supramolecular chemistry has long explored the balance ofnon-covalent interactions like hydrogen bonding, charge transfer (CT),and n-n stacking to develop novel functional materials. Several groupshave demonstrated the assembly of molecules into functionalone-dimensional (αD) structures with biological or electronicapplications (Hartgerink, J. D. et al., 2001 Science 294, 1684;Hartgerink, J. D. et al., 2002 Proceedings of the National Academy ofSciences of the United States of America 99, 5133; Silva, G. A. et al.,2004 Science 303, 1352; Hill, J. et al., 2004 Science 304, 1481 (2004);and Yamamoto, Y. et al., 2006 Science 314, 1761). Electrondonor-acceptor complex crystals, which also form 1D assemblies, exhibitproperties such as metallic conduction, ferroelectricity, and magnetism,whereby these attributes result from the electron transfer from anelectron rich donor to an electron poor acceptor along one dimension(Alves, H. et al., 2008 Nat Mater 7, 574; Collet, E. et al., 2003Science 300, 612; and Jain, R. et al., 2007 Nature 445, 291). Generally,organic CT complexes form a mixed stack or segregated stack lattice inwhich donors and acceptors assemble into face-to-face or edge-to-edgepairs, respectively. However, few purely organic molecular systems withhigher dimensionality have been observed whereby electron density may beshared in orthogonal dimensions (Moody, G. J. et al., 1987 Angew ChemInt Edit 26, 890; and Ashton, P. R. et al., 1994 J Chem Soc Chem Comm,181).

It is therefore desirable to provide a two-dimensional (2D)supramolecular network of charge transfer complexes that exhibitbidirectional charge transfer and monodomain visible pleochroism.

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide organic charge-transfer (CT) co-crystals into a crossed stacksystem, wherein the co-crystal exhibits monodomain visible pleochroism.

Accordingly, it will be understood by those skilled in the art that oneor more aspects of this invention can meet certain objectives, while oneor more other aspects can meet certain other objectives. Each objectivemay not apply equally, in all its respects, to every aspect of thisinvention. As such, the following objects can be viewed in thealternative with respect to any one aspect of this invention.

Other objects, features, benefits and advantages of the presentinvention will be apparent from this summary and the followingdescriptions of certain embodiments, and will be readily apparent tothose skilled in the art. Such objects, features, benefits andadvantages will be apparent from the above as taken into conjunctionwith the accompanying examples, data, and all reasonable inferences tobe drawn therefrom. The disclosures in this application of all articlesand references, including patents, are incorporated herein by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-c depicts molecules and crystal structures of cross stackedmaterials as used herein; a) molecular structures of electron donor andelectron acceptor components; b) co-crystal 1α·7β; c) co-crystal 1α·8β.

FIGS. 2a-b provides polarized FT-IR spectrum of cross stacked a)co-crystal 1α·7β and b) co-crystal 1α·8β.

FIGS. 3a-g shows the pleochroic behavior of crossed stack LASO materialsdisclosed herein; a) a petrographic microscope used to image crossedstack materials; b) single crystal of co-crystal 1α·7β imaged in theone-polarizer mode; e) single crystal of co-crystal 1α·8β imaged in theone-polarizer mode; materials imaged in c) parallel polarized mode for1α·7β; d) cross polarized mode for 1α·7β; and f) parallel polarized modefor 1α·8β and g) cross polarized mode for 1α·8β.

FIGS. 4a-f is UV-Vis spectroscopy of pleochroic crystals; a), b) and c)single crystals of co-crystal 1α·7β; and d), e) and f) co-crystal 1α·8β,examined to elucidate absorption anisotropy; polarized UV-Visspectroscopy of 1α·7β and 1α·8β shows two distinct absorption peaksassociated with the intermolecular charge-transfer along the (1 0 0) andthe electronic transitions in the secondary edge-to-face acceptor.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, a non-limiting embodiment of the invention is an organiccharge-transfer (CT) in a crossed stack system exhibiting bidirectionalcharge transfer interactions where one donor molecule shares electronswith two different acceptors, wherein one acceptor is face-to-face andthe other is edge-to-face, and wherein the co-crystal exhibits visiblepleochroism with RGB (red, green and blue) and CMYK (cyan, magenta,yellow and key) colors. As disclosed in detail in U.S. Ser. No.13/476,974 (filed May 21, 2012) and the references disclosed therein,the application and cited references incorporated herein in theirentirety, the co-crystal consists essentially of an electron acceptormolecule (A) and an electron donor molecule (D), wherein one of A and Dis an α-complement and the other one of A and D is a β-complement, suchthat the β-complement is incorporated into the α-complement throughmolecular linkages in a solvent system to form a co-crystallinesupramolecular network, wherein one or more of the molecular linkagesbetween α-complement and the β-complement use adaptive intermolecularrecognition to form the one or more molecular linkages.

In a specific non-limiting example, the co-crystals disclosed herein (2Dstructures) incorporate naphthalene (7β, 8β) and pyromellitic diimide(1α) derivatives shown in FIG. 1a . CT co-crystals of compounds 1α·7βand 1α·8β (as numbered in U.S. Ser. No. 13/476,974) self-assemble fromsolution, without a template, using a liquid-liquid diffusion techniquein ambient conditions. The structures, which appear as macroscopic flatsheets (500 μm×500 μm), are studied by X-ray crystallography (FIG. 1band FIG. 1c ) and reveal an architecture for CT pairs termed herein as a“crossed stack” lattice. Referring to FIG. 1, when co-crystallized,compounds 1α·7β (FIG. 1b ) and 1α·8β (FIG. 1c ) form three-dimensionalsupramolecular crossed stack networks with H—O distances of 1.845 nm and1.843 nm, respectively. The atomic sites that participate insupramolecular interactions are depicted in dark and the hydrogen bondsbetween atoms are depicted as dark dashed lines (FIG. 1b and FIG. 1c ,inset). In the case of compound 1α·8β, the position of hydroxyl andamine arms on compound 8β is disordered within the crystal. For clarity,the arms are shown to be alternating along the mixed stack.

In this assembly, donor and acceptor molecules organize face-to-face,similar to a mixed stack; however, a second acceptor is orientededge-to-face with each donor (FIG. 1b and FIG. 1c , inset). In thecrossed stack network, and as used herein, the face-to-face stackingdimension is labeled the “mixed stack axis” and the edge-to-facedimension is the “crossed stack axis.” Four supramolecular forces areresponsible for the formation of a crossed stack network: (i) chargetransfer, (ii) π-π n stacking, (iii) hydrogen bonding, and (iv) van derWaals interactions. The pyromellitic diimide acceptor (1α) is capable ofhydrogen bonding with neighboring donors and acceptors through twodiethylene glycol supramolecular “arms.” Additionally, both symmetric(7β) and asymmetric (8β) donors are functionalized with short hydroxylor amino arms that can hydrogen bond with neighboring acceptors.Co-crystal 1α·8β incorporates an asymmetric donor with an intramoleculardipole. In the solid-state material, this dipole is randomly orientedwithin the lattice resulting in network-wide dipole disorder. Thismolecular design, called Lock-Arm Supramolecular Ordering (LASO),utilizes noncovalent interactions to form an ordered, close packed,solvent-free network.

The propinquity of an electron rich donor and electron poor acceptormolecules enables ground-state electron transfer processes. Ionicity (ρ)is a measure of the amount of CT within a system (Soos, Z. G., 2007 ChemPhys Lett 440, 87, incorporated herein by reference). Here, ρ isquantified for each material by using polarized vibrational spectroscopy(FT-IR). Shifts in the ungerade modes of molecules are used to calculateρ because they are decoupled from electron-molecular vibrationinteractions. A linear shift of the C═O stretch (1728-1716 cm⁻¹) is usedto calculate p along an axis for each compound (FIG. 2a and FIG. 2b ).Referring to FIG. 2, crossed stack materials are placed on a siliconwafer and illuminated under ambient conditions. Using polarized IRradiation, reflected light is measured to determine absorption. It isknown that the carbonyl region of compound 1α absorbs between 1700 and1800 cm⁻¹. The carbonyl peak shift from crossed stack materials iscompared with neutral and reduced pyromellitic diimide to determine theionicity of each LASO complex. Interestingly, both structures shareelectrons along the mixed stack (ρ_(ms)) and crossed stack (ρ_(xs)) axesof similar amounts. For compound 1α·7β, ρ_(ms)=0.53 and ρ_(xs)=0.47,while for compound 1α·8β, ρ_(ms)=0.57, ρ_(xs)=0.42. The anisotropy inionicity between the orthogonal mixed stack and cross stack directionsproves the two-dimensional charge transfer in the LASO system.Ionicities for the mixed stack and crossed stack axes are found to beapproximately along the (1 0 0) and (0 1 0), respectively. The mixedstack and crossed stack ionicities for both compounds are at the cusp ofthe neutral-ionic border (ρ=0.5) and the summation of the ρ_(ms) andρ_(xs) ionicities suggests that both naphthalene-based donors are nearlyfully ionized (Okamoto, H. et al., 1991 Phys Rev B 43, 8224,incorporated herein by reference). As used herein, the unique CTinteractions in orthogonal directions are described as bidirectional CT.Moreover, both structures violate the mutual exclusion rule of the IRand Raman modes along the mixed stack and crossed stack axes implying anon-centrosymmetric lattice. This conclusion can be drawn fromspectroscopic evidence, though X-ray crystallography can be refined inboth centrosymmetric and non-centrosymmetric space groups. With acceptor(1α), both symmetric (7β) and asymmetric (8β) donors formnon-centrosymmetric crystalline networks that have bidirectional CT.

The unique assembly and optical properties arising from CT interactionsare responsible for pleochroic behavior in these 2D materials. Whenplaced in a petrographic microscope setup, shown in FIG. 3a , the colorof a crystal changes with the polarization angle of incident light (FIG.3b and FIG. 3c ). Pleochroism is demonstrated in crossed stack materialswhen placed in a petrographic microscope without the analyzer (FIG. 3a). When rotated in free space, co-crystal 1α·7β exhibits orange, yellow,brown, purple, and pink hues (FIG. 3b ). Co-crystal 1α·8β exhibitspurple, pink, orange, and yellow in transmission (FIG. 3e ). Thistransmitted light represents the components of plane-polarized lightthat is not absorbed by the crystal. When a second polarizer (analyzer)is inserted compound 1α·7β exhibits orange, brown, green, blue, purple,pink, and red in the parallel orientation (FIG. 3c ) and purple, blue,green, yellow, orange, and pink in the cross orientation (FIG. 3d ).Compound 1α·8β exhibits purple, red, orange, and brown in the parallelorientation (FIG. 3f ) and orange, brown, green, turquoise, blue,purple, and pink in the cross orientation (FIG. 3g ).

The absorption of both crystalline networks is highly anisotropic.Polarized UV-Vis spectroscopy is performed to determine the origin ofabsorption anisotropy in these crossed stack networks. In eachco-crystal, two distinct chromophores (FIG. 4c and FIG. 4f ), resultingfrom different electronic dipole moments, are observed. Crystal 1α·7β(FIG. 4a and FIG. 4b ) preferentially absorbs light at approximately 53°and 95° with absorption maxima at 527 nm and 475 nm, respectively (FIG.4c ), while 1α·8β (FIG. 4d and FIG. 4e ) at approximately 89° and 117°with absorption maxima at 490 nm and 595 nm, respectively (FIG. 4f ).Though the UV-Vis data for co-crystal 1α·8β suggests only onechromophore, careful examination of the peak illustrates that theabsorption maxima (wavelength) of the absorption changes withpolarization angle, suggesting two overlapping chromophores.

The polarization angles for each chromophore can be linked to themolecular faces of each crossed stack network. By determining the unitcell of a specific crystal, indexing its faces, and performing polarizedUV-Vis, the strongest absorption (at 95° in 1-3 and 89° in 2-3) isassociated with the charge-transfer interaction between face-to-facedonor-acceptor complexes along the (1 0 0). The second chromophore, 42°or 28° away, arises from the transition dipole along the face of theedge-to-face electron acceptor.

This non-parallel absorption is highly unexpected as most crystals withmore than one absorption band typical have parallel or orthogonalabsorption maxima consistent with the crystal eigendirections. However,crossed stack crystals are very thin, the retardance is first order, andthe vibration directions are not manifest. Moreover, in the triclinicsystem the eigendirections are not fixed by any symmetry. Thickness ofLASO crystals that exhibit pleochroic behavior range from 1-12 μm (SOM).To further verify the presence of two absorbers, the dichroic ratio iscalculated at each absorption maxima for each crystal and is found to besignificantly different (SOM). Given that mixed stack materials absorbstrongly in the visible spectrum, the two absorptive axes in co-crystal1α·7β and 1α·8β correspond to the mixed stack axis and opticaltransitions in the crossed stack acceptor.

The assembly that enables these electronic and optical phenomena resultsfrom the competition between π-π stacking, CT and hydrogen bonding.Compound 1α forms intermolecular H-bonds through two flexible glycolappendages (arms) and four carbonyl moieties, while the CT complements7β and 8β rely on shorter amino and hydroxyl arms for binding torecognition sites. In the solid state, hydrogen bonding occurs betweenthe glycol arms and the rigid arms of the donor (FIG. 1b and FIG. 1c ),effectively stabilizing the crossed stack assembly. In conventional CTsystems, the π-π stacking and CT would encourage faster growth along onedimension yielding a mixed stack or segregated stack morphology;however, the competition between hydrogen bonding and CT locks thestructure into an unexpected architecture. The balanced noncovalentinteractions, brought about by conformationally flexible glycol arms,result in closely packed, solvent-free, sheet-like co-crystals. The 2-Darchitecture of these materials is not a traditional mixed stackco-crystal, where van der Waals forces are responsible for holding theDA stacks together (Saito, G. et al., 2007 Bulletin of the ChemicalSociety of Japan 80, 1, incorporated herein by reference). Regarding thelattice structure, the material is a hybrid supramolecular architecturethat incorporates a 3D hydrogen bonded network and two uniquealternating CT stacks into a single lattice and is better classified asa network solid.

The disclosures of all articles and references, including patents, areincorporated herein by reference. The invention and the manner andprocess of making and using it are now described in such full, clear,concise and exact terms as to enable any person skilled in the art towhich it pertains, to make and use the same. All references cited inthis specification are incorporated herein by reference. It is to beunderstood that the foregoing describes preferred embodiments of thepresent invention and that modifications may be made therein withoutdeparting from the spirit or scope of the present invention.

What is claimed is:
 1. An organic charge-transfer (CT) co-crystal in acrossed stack lattice exhibiting bidirectional CT interactions, whereone donor molecule shares electrons with two different acceptors;wherein one acceptor is face-to-face with the donor and a secondacceptor is edge-to-face with the donor, said organic CT co-crystalselected from the group consisting of


2. The organic CT co-crystal according to claim 1, wherein the ionicityof

are ρ_(ms)=0.53, ρ_(xs)=0.47, and ρ_(ms)=0.57, ρ_(xs)=0.42,respectively.
 3. The organic CT co-crystal according to claim 1, whereinthe co-crystal violates the mutual exclusion rule of IR and Raman modesalong a mixed stack axes and a crossed stack axes.
 4. The organic CTco-crystal according to claim 1, wherein absorption of light of theco-crystal is anisotropic.
 5. The organic CT co-crystal according toclaim 4, wherein two distinct chromophores are observed.
 6. The organicCT co-crystal according to claim 5, wherein the two distinctchromophores absorb light at 53° and 95° with absorption maxima at 475nm and 527 nm, respectively.
 7. The organic CT co-crystal according toclaim 5, wherein the two distinct chromophores absorb light at 89° and117° with absorption maxima at 490 nm and 595 nm, respectively.